Sleeping on the back or right side, rather than the left, doubled the risk - but only to almost four in 1,000.

Left-side lying aids blood flow to the baby, as the mother's major blood vessels are unimpeded by a heavy womb.

The New Zealand study, published in the British Medical Journal, called for larger studies to test the findings.

Ms Daghni Rajasingam of the Royal College of Obstetricians and Gynaecologists said: "There are many factors which are linked to still-birth including obesity, increasing maternal age, ethnicity, congenital anomalies and placental conditions. A significant number are unexplained.

We would like to see further research into sleep in pregnancy encouraged and funded as a matter of urgencyJanet Scott of the stillbirth charity Sands

"This small-scale study looks at another possible factor. However, more research is needed into sleep patterns before any firm conclusions over sleeping positions can be made.

"In the meantime, women should speak to their midwives if they are concerned."

The UK has one of the highest still-birth rates in the developed world. Every year here 4,000 babies are still-born.

A third of still-births have no clear cause.

Janet Scott, of the still-birth and neonatal-death charity Sands, said: "We would like to see further research into sleep in pregnancy encouraged and funded as a matter of urgency.

"The study will require further validation before any widespread public health campaign could be justified.

"Mums want to know what they can do to reduce the chance of this happening to their baby.

"A simple message which mums could follow, which would reduce their risk of still-birth, would be very welcome."